Considering a programmable logic device (PLD) as one example of an integrated circuit device, as applications for which PLDs are used increase in complexity, it has become more common to design PLDs to include configurable specialized processing blocks in addition to blocks of generic programmable logic resources. Such configurable specialized processing blocks may include a concentration of circuitry on a PLD that has been partly or fully hardwired to perform one or more specific tasks, such as a logical or a mathematical operation. A configurable specialized processing block may also contain one or more specialized structures, such as an array of configurable memory elements. Examples of structures that are commonly implemented in such configurable specialized processing blocks include: multipliers, arithmetic logic units (ALUs), barrel-shifters, various memory elements (such as FIFO/LIFO/SIPO/RAM/ROM/CAM blocks and register files), AND/NAND/OR/NOR arrays, etc., or combinations thereof.
These fixed-logic elements within the configurable specialized processing blocks are interconnected by a configurable interconnect structure within the configurable specialized processing block. They may also be able to accept parameters as well as data inputs. Thus, while the elements are fixed in the type of arithmetic or logical functions that they perform, their interconnection within the block is flexible under user control, and moreover, if an element accepts parameters, then the way in which it performs its fixed function may be subject to a degree of user control. In addition, it may be possible to route the outputs of some or all of the fixed-logic elements within a block either to another fixed-logic element within the block or directly out of the block.
One particularly useful type of configurable specialized processing block that has been provided on PLDs is a digital signal processing (DSP) block, which may be used to process, e.g., audio signals. Such blocks are frequently also referred to as multiply-accumulate (“MAC”) blocks, because they include structures to perform multiplication operations, and sums and/or accumulations of multiplication operations.
For example, PLDs sold by Altera Corporation, of San Jose, Calif., as part of the STRATIX® and ARRIA® families include DSP blocks, each of which includes a plurality of multipliers. Each of those DSP blocks also includes adders and registers, as well as programmable connectors (e.g., multiplexers) that allow the various components of the block to be configured in different ways.
Typically, the arithmetic operators (adders and multipliers) in such configurable specialized processing blocks have been fixed-point operators. If floating-point operators were needed, the user would construct them outside the configurable specialized processing block using general-purpose programmable logic of the device, or using a combination of the fixed-point operators inside the configurable specialized processing block with additional logic in the general-purpose programmable logic.